The invention relates to a valve assembly that is designed for relieving pressure on a process structure, such as a reactor vessel.
In chemical process plants many different types of pressure relief systems are installed on pressure vessels, such as reactors or storage tanks, or on other structures that carry fluids under high pressure, such as pipelines. Many of the pressure relief systems use a double arrangement of pressure relief valves and associated piping to provide a safety factor for relieving an overpressure condition. But, some of these systems are not satisfactory from the standpoint of safety. The unsafe condition is usually created when an operator makes a mistake in his sequence for opening or closing the valve on the inlet or discharge side of the pressure relief valves.
As an example of a typical double-valve pressure relief system, a three-way valve is mounted on the outlet fitting of a reactor. The other two ports in the three-way valve are each connected into the inlet side of a pressure relief valve. An outlet valve or Hamer blind is connected into the discharge side of each pressure relief valve, and each outlet valve is connected by appropriate piping into a flare header or some other vent system. After the system has been installed, it may be necessary to isolate the pressure relief valves, one at a time, from the reactor and the flare header to perform routine maintenance operations, such as replacing or repairing the relief valve or other parts. The isolation step involves aligning the three-way valve to the appropriate pressure relief valve and closing the outlet valve or Hamer blind on the discharge side of the other pressure relief valve, so that the relief valve is isolated from both the reactor and vent header.
After the repair and/or replacement job is completed the outlet valve or Hamer blind is opened, and the three-way valve is aligned to the pressure relief valve that has been repaired. Fluid in the reactor can then pass through the three-way valve, the pressure relief valve, and the outlet valve or Hamer blind into the flare header, if an overpressure condition should develop in the reactor. But, if the operator should forget to open the outlet valve, or the Hamer blind, a hazardous situation can occur. The hazard is created by the fact that the pressurized fluid cannot flow through the other pressure relief valve and the outlet valve, because the port in the three-way valve that communicates with the other pressure relief valve is closed.
The use of a Hamer blind to isolate the pressure relief valve has another drawback. It is a manual operation in which wrenches and other hand tools must be used, and air usually gets into the lines when the flange bolts are loosened to insert the blind. If the air mixes with highly flammable gases, such as methane or propane, it can result in explosion and fire. Another disadvantage of the system described above is the use of a three-way valve ahead of the pressure relief valve. When fluid flows through a three-way valve it must follow a tortuous path and the pressure drop is quite substantial.
The present invention overcomes most of the problems described above. The invention provides a valve assembly which includes two pressure relief valves, each being positioned between an inlet valve and an outlet valve. A single mechanical actuator means is used to open and close both pairs of valves, so that when one pair of inlet and outlet valves is fully closed, the other pair of inlet and outlet valves is fully open.